Until a recent date, widely utilized white light-emitting LEDs have been of a double color admixture type of which white light is produced by mixing a blue light released from a semiconductor light-emitting element upon activation by electric energy and a yellow light emitted from a yellow light-emitting phosphor upon excitation with the blue light from the semiconductor light-emitting element. However, there is a problem in the white light emitted from the white light-emitting LED of the double color admixture type, in that the white light is not sufficiently pure. Therefore, there have been studied white light-emitting LEDs of a triple color admixture type which give a white light by mixing a blue light, a green light and a red light. The blue light, green light and red light are all released respectively from a blue light-emitting phosphor, a green light-emitting phosphor and a red light-emitting phosphor by excitation with a light in the wavelength region of 350 to 430 nm which is released from a semiconductor light-emitting element by applying thereto electric energy.
As the green light-emitting phosphor, there have been known green light-emitting silicate phosphors such as an alkaline earth metal silicate phosphor (e.g., strontium barium silicate) activated with europium.
For example, Patent publication 1 (JP 2008-88399 A) discloses a green light-emitting silicate phosphor having the following formula:(MI(1-x)MIIx)αSiOβwherein MI is at least one element selected from the group consisting of Ba, Ca, Sr, Zn and Mg; MII is at least one divalent or trivalent metal element; each of x, α and β satisfies the conditions of 0.01<x<0.3, 1.5≦α≦2.5, and 3.5≦β≦4.5.
Patent publication 1 describes that MI preferably comprises Ba and Sr and that [Ba] and [Sr] preferably satisfy the conditions of 0.5<{[Ba]/([Ba]+[Sr])}≦1, wherein [Ba] is a molar ratio of Ba based on the total mol of Ba and Sr comprised in MI and [Sr] is a molar ratio of Sr based on the total mol of Ba and Sr.
Patent publication 2 (JP 2007-238814 A) discloses that a green light-emitting phosphor having the formula of SrxBa2-xSiO4:Eu (0≦x≦2) shows increased excitation efficiency in the blue light-emission region when a magnesium salt is added. The magnesium salts described in Patent publication 2 are magnesium chloride, magnesium sulfate, and magnesium fluoride.
Patent publication 3 (JP 2010-3790 A) has a description referring to the working temperatures of LED. In more detail, it is described that the LED chip (i.e., semiconductor light-emitting element) generates so heat that the temperature of LED chip reaches 120° C., when the LED is continuously operated for a long period of time or activated by large electric current for the purpose of increasing the light-emitting strength. Therefore, it is required that the phosphor for the use as a light-emitting source of a white light-emitting LED shows a light emission not substantially varying with the temperature variation caused by the heat generation caused during the operation of LED. In other words, the phosphor should be stable at elevated temperatures in the vicinity of 120° C.